1. Field of the Invention
The present invention relates to a method of changing over a path switch in an optical transmission device which transmits and receives SONET (Synchronous Optical Network) signals in an A/B ring mode or a linear mode, and more particularly to a method of changing over a path switch which is connected, through respective paths, to two LTE (Line Terminal Equipment) units in an optical transmission device which receives optical signals transmitted bidirectionally over optical transmission lines, for selectively connecting one of the LTE to a lower-level receiver.
2. Description of the Related Art
The recent development of an information society results in demands for communicating information in a wide range of forms (e.g., speech, images, data) and in a greater quantity over a wider geographic region. To meet such demands, efforts are being made to establish global standards for communication interfaces in various organizations including the CCITT (International Telegraph and Telephone Consulative Committee). One of the proposals is SONET which is optical interface standards in North America (including the United States of America). There are attempts to design and develop optical transmission devices according to the SONET, referred to as FLM (Fiber Looped Multiplexer) 150/600 ADM (Add/Drop Multiplexer).
To perform ADM operation of the FLM 150/600 ADM, bidirectional paths therein have to be selected or changed over by a path switch.
An FLM 150/600 ADM for transmitting and receiving SONET signals operates in an A/B ring mode as follows: Two identical optical signals that have been bidirectionally transmitted from a transmitting ADM over ring-shaped transmission lines are received by a receiving ADM, and the received signals or a signal free of a line fault is selected by the path switch, and transmitted to a lower-level receiver.
According to the standards TA (TECHNICAL ADVISORY)-496, third volume, pushed by BELLCORE (Bell Communications Research), which is a U.S. organization for establishing communication standards, the path switch selects an error-free signal when:
1. RINDWN is produced; PA1 2. P-AIS (Path-Alarm Indication Signal) is received; or PA1 3. B3-Error is generated. PA1 1. the above three faults occur simultaneously with respect to both two signals that are supplied from the two LTE through the respective paths to the path switch; PA1 2. the above three faults occur successively, resulting in a frequent changeover of the path switch; PA1 3. the path switch is to be manually changed over in the absence of the above three faults, in order to confirm whether the path switch can normally be changed over; PA1 4. the path switch is to be locked to a selected one of the paths for transmission and reception of optical signals over a unidirectional transmission line of the ring-shaped transmission lines, without concern over the above three faults; or PA1 5. the path switch is to be forcibly changed over for a test even if any one of the above three faults is taking place.
RINDWN is an alarm signal that is produced indicating an input signal fault when no clock is supplied while the clock of a signal (composed of data and clock) supplied to the path switch is being monitored. P-AIS is an alarm signal received by the path switch, which is carried on a transmitted signal in the event of a fault of a higher-level side. Higher-level faults include lack of an input signal for a higher-level transmitter/receiver, an optical fiber break, a bit error, etc. B3-Error is an alarm signal indicating an input signal fault when an error rate of signals supplied to the path switch exceeds 0.1%.
When either one of the above three alarm signals is produced, the path switch connects the path other than the path which is suffering the fault to a lower-level receiver.
When an FLM 150/600 ADM for transmitting and receiving SONET signals operates in linear mode, the path switch is changed over based on only an un-eq (Un-Equipment) code contained in each of signals that are bidirectionally supplied to the ADM. The un-eq code is a code capable of determining whether a signal is being transmitted from a transmitting ADM. When all un-eq codes are 0, it is determined that no signal is being transmitted from the transmitting ADM, and a signal from another ADM than the ADM from which the un-eq codes are supplied is selected by the path switch and transmitted to a lower-level receiver.
However, there has not been developed any technique as to how to operate the path switch during operation of an ADM operates in the A/B ring mode, when:
Therefore, it has been impossible to operate the path switch when one of the above five conditions occur.
Furthermore, while an ADM is operating in the linear mode, there has been no way to meet a demand for transmission and reception of optical signals over a unidirectional transmission line of the ring-shaped transmission lines.